Mold neutralization requires powerful disinfection. Chlorine bleach is likely the most powerful disinfectant you will find, and the CDC recommends diluted bleach for general mold-killing applications. Due to its concentrated power, it requires excessive water dilution. Add a full gallon of water to every cup of bleach that you will require. If you prefer a more natural approach, home improvement expert Tim Carter recommends a natural chlorine bleach solution: oxygen bleach. Use oxygen bleach powder and add hot water as advised by the manufacturer.
Due to the porosity of wood and composite materials, never pour your bleach solution directly onto the deck. It will absorb the liquid and possibly cause warping. Apply the diluted bleach solution to your deck using a mop. If using oxygen bleach, let it sit on the deck surface for about an hour, because oxygen bleach works much slower than chlorine bleach and gradually releases oxygen ions. Scrub moldy areas using a firm scrubbing brush and then rinse the bleaching agent away with a hose.
If you have wood steps, apply the bleach solution using a damp cloth or spray bottle. If you have traditional concrete steps, you can pour the bleach directly onto the surface, since concrete is non-porous. Scrub the steps with a firm scrubbing brush to remove the mold, then rinse the steps with a hose. If any mold remains, add a bit more bleach solution and scrub a second time.
Keep wood and composite decking dry to prevent mold growth. Wood and composite materials unfortunately contain organic matter, and can support mold growth by their very nature. If you have extremely humid weather, paint over the decking with a mold-resistant sealant or paint. Concrete contains no organic matter, so just keep your concrete steps clean in order to prevent mold growth. As long as dirt does not have the chance to accumulate, no mold can grow.